Captain Thiago Silva insisted it was "not the real Paris St Germain" after his side suffered an early first defeat of the new Ligue 1 campaign as Rennes came from behind to win 2-1 at Roazhon Park.
PSG had taken the lead in the 36th minute when Edinson Cavani knocked in a stray pass across the penalty area from Rennes captain Damien Da Silva.
However, the hosts rallied and drew level just before half-time through a fine shot on the turn by M'Baye Niang.
Rennes made a fast start to the second half as Romain Del Castillo headed in after 48 minutes and they could have added another when Jeremy Morel's volley came back off the post.
PSG – again with Neymar left out of the squad as speculation continues over his future – offered few clear-cut chances in the second half.
Rennes, though, continued their 100 per cent start to the new campaign and sit third on six points alongside Lyon and Nice.
Brazilian defender Silva maintained the PSG squad will regroup.
"It was a difficult match, it's hard to talk, we started well and then we lost our concentration a little bit, which is a shame," he said on French broadcaster Canal+.
"At the beginning of the second half, it's a goal we do not normally concede, but we were not attentive.
"Sometimes it is like that, but this was not the real Paris St Germain that we know.
"We have to look forward, think about the next game, try to be better and forget everything that happened here.
"It's the beginning of the championship, they were stronger. Now, we have to try to improve and prepare well."
Neymar was again conspicuous by his absence from the PSG squad, with both former club Barcelona and Spanish rivals Real Madrid reportedly looking to push through a deal ahead of the transfer deadline.
PSG head coach Thomas Tuchel, though, maintains the uncertainty surrounding the Brazilian cannot be used as an excuse for Sunday's defeat.
"Neymar situation weighed on the team? No, I do not think so," he said to Canal +.
"It is still the same, but the transfer window is still open for more than 10 days.
"Do you think this thing will be settled tomorrow or the day after tomorrow? I do not think so.
"But no matter what I think things are not settled for the moment, and it is not possible for him to play because he has not done any training like the others, the risk of injury is too great."
Tuchel accepted his team had come up short of the required performance levels.
"We had problems in the challenges, both offensively and defensively," he said.
"We forgot how to close out the match after taking the first goal and then make a gift to Rennes before half-time.
"It is not an easy situation, many of our players are not 100 per cent (fit), we need time."
Rennes coach Julien Stephan felt his side were worth their victory, but can still improve the squad.
"We have a team that has shown interesting values, but to be able to last over time this season we will need some more people," he said. "I know our recruiting team is working to improve all that."